Dynamics of an obligate pollination mutualism in the Australian Phyllanthaceae

Abstract

Obligate pollination mutualisms (OPMs) are highly specialised pollination interactions in which both plant and pollinator are entirely dependent upon each other for reproduction. In general, OPM pollinators transport pollen between the male and female flowers of a single host species. OPM pollinators deposit eggs within the flowers that they fertilise and pollinator larvae develop by feeding upon the ovules fertilised by their mother. Thereby, plants involved in OPMs sacrifice a proportion of their offspring for a highly specific pollination service. In return for this service pollinators in OPMs gain a reliable food source for their developing offspring. The fruits of plants involved in OPMs may also host a diverse assemblage of non-pollinating seed parasites and parasitoids and these species may have important effects on the mutualism. In 2003 an entirely new pollination mutualism was described between moths in the genus Epicephala (Gracillariidae) and three species of plant in the genus Glochidion (Phyllanthaceae). Since their initial discovery, OPMs have been documented in several species of two additional genera within the Phyllanthaceae; Breynia and Phyllanthus. OPMs involving Epicephala moths are now believed to occur in approximately 500 species of the pantropical tribe Phyllantheae, making it the second largest OPM group after the fig-fig wasp radiation (>750 species). To date relatively few of the Epicephala-Phyllantheae OPMs have been studied. Of those species, even fewer have been studied over wide geographic areas and as such we may have a limited view of Epicephala diversity. Our understanding of the ecology of many of these plant species and their pollinators also remains limited. While mutualisms by definition benefit both parties involved in the interaction, conflicts of interest are predicted to occur between those groups. In OPMs, pollinators should theoretically maximise their fitness by saturating oviposition sites with eggs, resulting in very low rates of viable seed production and potentially the collapse of the mutualism. As in other mutualisms, mechanisms must exist to prevent pollinators in an OPM from overexploiting their host plant. However, for many host plants, we know nothing of the mechanisms that might prevent mutualism instability. This thesis aimed to address these outstanding issues and to increase our knowledge of the ecology of OPMs. In completing this thesis, I have discovered previously unknown diversity in the pollinators, non-pollinating seed parasites and parasitoids inhabiting the fruits of B. oblongifolia, highlighting the importance of temporal and spatial sampling effort in understanding interactions in OPMs. Although I made attempts to understand how the OPM in B. oblongifolia remains stable over evolutionary time, the exact mechanisms remain unknown. It is my hope that this thesis will enable future research on B. oblongifolia in order to address this question. Using a broad variety of experimental techniques and molecular tools, I have learnt a much about the OPM occurring in B. oblongifolia. As a consequence, our knowledge of the plant, its ecology and the community of pollinators, non-pollinating seed parasites and parasitoids that inhabit its fruits has been greatly improved. Several important questions remain, particularly in relation to mutualism stability and the evolutionary origins of the pollinators on B. oblongifolia. However, I believe that the discoveries made here will allow these questions to be answered in due time. Furthermore, I am in no doubt that this thesis will facilitate future research in Breynia and other OPMs, enabling others to ask further questions about the processes that shape mutualisms. In this way I hope this fascinating but overlooked plant and its equally unappreciated pollinators will gain the broader scientific recognition they so readily deserve.

Date of Award	2018
Original language	English